1. Field of the Invention
The instant disclosure relates to a direct current-direct current (DC-DC) converter, and more particularly, to a control device for the DC-DC converter and a method controlling the same.
2. Description of Related Art
Referring to FIG. 1A, FIG. 1A is a simplified block diagram of a traditional voltage/current feedback circuit in an average current control mode. Traditionally, in order to enable a power converter to perform the current control in a stable manner and with better/quicker transient response, both current feedback control circuit and voltage feedback control circuit are incorporated into the conventional DC-DC converter. As shown in FIG. 1A, an error amplifier 101 compares an output voltage Vout with a reference voltage signal Vref, and outputs a feedback signal Vcom, which is used as a reference current signal Iref for an input current Vin (in other words, Vcom=Iref). A control circuit 103 converts a current feedback control signal Vcontrol outputted by the error amplifier 102, which is a control voltage, into a switching signal SS.
Referring to FIG. 1B, FIG. 1B is a simplified lock diagram showing another traditional voltage/current feedback circuit, which is capable of performing the power factor correction. To be capable of performing the power factor correction, the reference current signal Iref in FIG. 1A is outputted by a multiplier 205, and the variation in the reference current signal is dictated by the feedback of the output voltage Vout to the input voltage Vin. For example, when the output voltage Vout is the given value, and the input voltage Vin is a rectified city power sinusoidal wave, the reference current signal could become another rectified sinusoidal wave. Accordingly, the converter in addition to functioning as the DC-DC converter can be capable of performing the power factor correction.
When it comes to current feedback control, a peak current mode-based approach and an average current mode-based approach are usually employed. In the peak current mode-based approach, extra slope compensation for the current may require when the duty cycle of the conduction of the switch is larger than 50%, which may lead to system instability. When the same peak current mode approach is applied in the power factor correction using the discontinuous conduction mode, the input current waveform may be distorted, resulting in the portion of the harmonic waves of different orders in the input current waveform staying too large. However, the use of the average current mode-based approach does not accompany with the aforementioned problems, and therefore is better suited to the DC-DC converters and the power factor correctors in different conduction modes.
When the power factor corrector operates in the current control mode, the control circuit often uses a multiplier to generate the current reference value, as shown in FIG. 1B. But of the expense, the complexity, and the inherent bandwidth limitation of the multiplier, a sampling and holding (S/H) circuit is proposed to replace the multiplier according to “A general technique for derivation of average current mode control laws for single-phase power-factor-correction circuits without input voltage sensing” by J. Rajagopalan, F. C. Lee and P. Nora in IEEE Transactions on Power Electronics, Vol. 14, No. 4, pp. 663-672, July 1999. This particular proposed approach when used in the DC-DC converters could perform as desired in a continuous conduction mode (CCM), critical mode (CRM) and discontinuous conduction mode. However, when used in power factor correction context, for maintaining the current wave in the desired form and the power factor, the power factor corrector having the proposed approach incorporated may need to be operating in the CCM. Meanwhile, in order to minimize the switching energy loss associated with the switching of the switching elements (e.g., switches and diodes) and therefore improve the conversion efficiency. The power converters may have to operate more in the CRM and the discontinuous conduction mode. Therefore, a novel power converter control circuit (or device) in design is still in need.